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Holoprosencephaly

Holoprosencephaly is a condition where the cerebral hemispheres are absent. Basically you end up with one cavity and there may be arrhinencephaly and there is cleft lip, again a midline defect. And these patients have severe mental retardation and motor deficits, seizures. Lissencephaly is another migrational error. It is often called by different names. Most commonly it would be called agyria. What happens here is it’s a disorder of sulcation, another term.

There are heterotopias in this condition, frequently, and the heterotopias being neurons that are not in the topology where they belong. It’s not ectopic because it’s in the same organ, but it is heterotopic because it is not normal gray matter. One of the genetics syndromes, called Miller-Decker syndrome, involves lissencephaly, one of the malformations. It also has other associated dysmorphology.

Chiari malformations. Type I Chiari is one where there is herniation of the cerebellar tonsils. That’s a very classic type of description, many of you probably recall. Frequently Chiari’s are asymptomatic until later. Not everyone presents with major symptoms right away. Type II includes what’s said in type I, but there is an association of other anomalies. There is an elongation of caudal medulla, which is an important finding, and there is a kinking of the cervical medullary junction and type II always implies also a lumbosacral meningomyelocele. Now if you go back several slides earlier you know exactly what a meningomyelocele is. It’s like a meningo and cephalocele but only myelo because it’s a spinal cord rather than encephalo.

Polymicrogyria is often due to some sort of acquired insult where the brain has a few too many neurons and there are too many gyri that are very very small. And it usually results in pretty severe seizure disorders. There is schizencephaly where there is a gray matter line cleft that may extend clear across the brain, from the pial lining to the ependymal lining. You may recall that the ependyma are the cells that line the ventricle. Agenesis of the corpus callosum is a fairly reasonably commonly encountered anomaly. It can be isolated, in which case you may not even know that a person has agenesis of the corpus callosum.

Porencephaly is not really often a malformation. This can often be simply the residue of a small stroke. It is just a CSF-filled space where there ought to be brain. Should be distinguished from hydranencephaly. Hydranencephaly is a congenital absence of the cerebral hemispheres and we are not fully sure how this comes about, but it is very striking when you look at an MRI or a CT.